There are several new features and functions added by Apple in latest iOS 12.1.1 final update with bug fixes and performance improvements, which you can read in below changelog image.

watchOS 5.1.2 Changelog:

How to Offline Install iOS 12.1.1 Final Update?

If you are using WiFi connection at home or office or if you have got unlimited internet data bundle of 4G LTE or 3G from your network provider, then you can simply navigate to Settings >> General >> Software Update option for latest iOS 12.1.1 OTA Update online installation. In case if you want to perform manual iOS 12.1.1 offline installation using iTunes software then you can free download iOS 12.1.1 Update .ipsw files through direct links available below from Apple's server.